Field of the Invention
Rigid polyurethane foams are well known and are commonly prepared from organic polyisocyanates and organic polyols together with known blowing agents, surfactants and catalysts for the reaction of --OH and --NCO groups. Such foams are used in construction, refrigeration and insulation applications because they may be prepared in a wide variety of densities and because they are closed cell foams.
Perhaps the most critical requirements for rigid foams for use in building panels are the combustibility standards imposed by the various building codes in the United States. The basic combustibility test for the foam is ASTM E-84. Rigid foam used in metal faced building panels is required to pass ASTM E-84 with a Class 1 rating, flamespread of 25 or less, and smoke of less than 450. In general, panels produced on continuous laminators utilize Class 1 rated polyisocyanurate foam systems. While such systems provide high line speed and short dwell times, such systems typically have narrow processing windows. One example of this is the sensitivity of such systems to small changes in processing temperatures. In comparison, polyurethane foams are not sensitive to small changes in processing temperatures and have better adhesion to metal.
In order to improve the ASTM E-84 performance of foams, aromatic polyester polyols have been used in the production of polyurethane foams and urethane modified polyisocyanurate foams. The use of such polyester polyols is described, for example, in U.S. Pats. Nos. 4,544,679 and 4,797,428, and the various references cited therein. See also the Hercules technical data bulletin numbers S166A (dated December 1977) and OR-255 (dated April 1980), and the Mobil Chemical product bulletin dated 11-20-78.
Finally, in a paper entitled "Polyester Polyols in Rigid Polyurethane and Polyisocyanurate Foams in Structural Building Panels", presented at the Polyurethanes World Congress in late 1987, the authors therein concluded that aromatic content, functionality, free glycol content and hydroxyl number are the four key variables of polyester polyols affecting the ASTM E-84 classification of polyisocyanurate and polyurethane foams for building panels.
It was a primary object of the present invention to provide rigid polyurethane (PU) foams having an ASTM E-84 Class 1 rating as well as desirable physical and insulation properties which foams may be produced under a variety of processing conditions.